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Shape-Memory Nanopores Induced in Coordination Frameworks by Crystal Downsizing

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Science  11 Jan 2013:
Vol. 339, Issue 6116, pp. 193-196
DOI: 10.1126/science.1231451

Size Affects Shape

Porous molecular framework materials can adopt a different phase when guest molecules absorb and uniformly distort the framework. Usually the framework returns to its original shape when the guests desorb. Sakata et al. (p. 193) noted that because surface stress drives this process, it might be avoided in smaller crystals. Indeed, a flexible porous coordination polymer, [Cu2(dicarboxylate)2(amine)]n, could retain the structure induced by guest molecules such as methanol if crystallites were made sufficiently small (submicrometer scale) and did so to a greater degree as the crystallite dimensions decreased.

Abstract

Flexible porous coordination polymers change their structure in response to molecular incorporation but recover their original configuration after the guest has been removed. We demonstrated that the crystal downsizing of twofold interpenetrated frameworks of [Cu2(dicarboxylate)2(amine)]n regulates the structural flexibility and induces a shape-memory effect in the coordination frameworks. In addition to the two structures that contribute to the sorption process (that is, a nonporous closed phase and a guest-included open phase), we isolated an unusual, metastable open dried phase when downsizing the crystals to the mesoscale, and the closed phase was recovered by thermal treatment. Crystal downsizing suppressed the structural mobility and stabilized the open dried phase. The successful isolation of two interconvertible empty phases, the closed phase and the open dried phase, provided switchable sorption properties with or without gate-opening behavior.

Shape-memory materials alter their morphological appearance in response to an external stimulus (for example, mechanical stress created by macroscopic structural deformation), hold their new temporary shape after the stimulus has been removed, and return to their original morphology in the presence of another external stimulus (1, 2). For instance, a metal alloy can exhibit shape-memory effect if it has two phases that can interconvert reversibly; that is, without requiring atoms to diffuse through the structure. Here, we describe a molecular-scale shape-memory effect (MSME) in nanoporous framework materials in which the application of an adsorption stress deforms the original shape of the nanopore into a temporary shape, which is maintained even after desorption, and thermal treatment then recovers the original shape.

Our design takes advantage of the flexibility of crystalline porous coordination polymers (PCPs) (38), which are assembled from organic spokes and inorganic joints. These flexible PCPs cooperatively reconfigure their framework structures in response to the incorporation of molecules into the nanopores; this adsorption process triggers the deformation of the pore shape. Most flexible PCPs recover the original structure after the removal of the adsorption stress (that is, the desorption of the guest molecules), which leads to the so-called framework elasticity property (9). The MSME requires that any structural transformation during desorption should be suppressed. We show that crystal downsizing influences the structural mobility, because a reduction in the number of repeating units should be sufficient to regulate the cooperative nature of the structural transformation and the effect of stress.

We fabricated MSME nanopores by crystal downsizing, which regulated the flexibility of the framework, and demonstrated the switchable sorption events based on the presence of two interconvertible pore shapes (Fig. 1). Among the variety of flexible PCPs, we chose a PCP with a twofold interpenetrated framework (1012)—namely, [Cu2(bdc)2(bpy)]n (1, bdc = 1,4-benzenedicarboxylate, bpy = 4,4′-bipyridine) (13)—that exhibits a cooperative structural transformation from the nonporous closed phase to the guest-included open phase with a gate-type abrupt sorption behavior (fig. S1) (14).

Fig. 1

Schematic illustration of the induction of a shape-memory effect in porous frameworks through crystal downsizing, which suppresses the structural mobility. The micrometer-sized crystals of the two-fold interpenetrated porous frameworks of [Cu2(bdc)2(bpy)]n (1) show the elastic framework flexibility in response to guest incorporation. By downsizing the crystal to the mesoscale, an empty open structure is isolated as an unusual, temporary deformed phase and is then converted to the original closed phase by thermal treatment. This novel structural flexibility is defined as the shape-memory effect in PCPs.

We confirmed the identities of both structural phases by single-crystal x-ray diffraction (XRD) for 1 and 1⊃G [G = N,N′-dimethylformamide (DMF), ethanol, or methanol] and that all phases crystallized as triclinic P-1. The framework structure is composed of two identical but distinct frameworks based on six connected dicopper paddlewheel nodes interconnected with bdc in the equatorial plane and bpy in the apical positions. In the open structure of 1⊃DMF, the two scaffolds are located near one another (3.42 Å) because of the presence of interframework π-π interactions that produce the accessible channels (Fig. 2, A and B; 1⊃methanol and 1⊃ethanol are shown in fig. S2). The removal of guest molecules leads to the closed phase of 1, in which the framework undergoes a drastic shearing to minimize the amount of void space and dislocates the center of the void while maintaining the overall framework connectivity (Fig. 2, C and D).

Fig. 2

The structural dynamics of 1 in response to the incorporation of guest molecules. (A and B) The crystal structure of the DMF-incorporated framework of 1⊃DMF illustrates the open framework structure: the standard structural unit of interpenetration (A) and the overall framework structure (B) with the view along the main axis of bdc. The guest DMF molecules are omitted for clarity. (C and D) The crystal structure of the guest-removal phase of 1 illustrates the closed framework structure without permanent porosity: the standard structural unit of interpenetration (C) and the overall framework structure (D) with the view along the main axis of bdc. One of the interpenetrated frameworks is highlighted in green and the other in purple. The van der Waals surface is highlighted in yellow.

Crystal downsizing of PCPs has previously been accomplished by rapid nucleation with the use of microwaves (15) or ultrasound (16) or by inverse microemulsion with surfactants (17). Our approach, coordination modulation (18), allows the coordination equilibrium at the crystal surface to be controlled with an additive with the same coordination moiety as the linker, which provides highly crystalline meso-sized crystals with a defined crystal surface structure. Micrometer-sized polycrystalline powders of 1 (1-micro) were synthesized by first forming a two-dimensional (2D) square grid of [Cu(bdc)(S)]n (S = solvent) by reacting copper ions with bdc. Insertion of bpy and an exchange reaction with S formed a 3D framework. Meso-sized crystals of 1 (1-meso) were made by adding acetic acid in the first step of the reaction as a modulator to a mixture of copper acetate and H2bdc (H2bdc = 1,4-benzenedicarboxylic acid).

As the modulator concentration ratio (r = 10, 20, 30, 40 and 50, where r = [acetic acid]/[copper acetate]) was increased, the average crystal size of 1-meso increased from ~50 by 50 by 20 nm3 (1-meso50; r = 10) to ~300 by 300 by 30 nm3 (1-meso300; r = 50) (Fig. 3, A to F; figs. S3 to S5; and table S1) (19). Electron diffraction patterns indicated that the shortest dimension of the platelike crystals of 1-meso corresponded to the Cu-N coordination direction of the framework (fig. S6). Whereas all as-synthesized crystals were identified as the guest-included open phase by powder XRD analysis, the subsequent evacuation of those crystals led to the formation of different structures, depending on crystal sizes (Fig. 3G and fig. S7). As observed in a single crystal of 1, powders of 1-micro and the largest meso-sized crystal (1-meso300) formed the closed phase. In contrast, the smallest crystal of 1-meso50 maintained the open phase, even after the removal of the guest molecules under vacuum (the open dried phase).

Fig. 3

(A to E) Transmission electron microscopy (TEM) images of meso-sized crystals. (A) 1-meso50, (B) 1-meso60, (C) 1-meso110, (D) 1-meso160, and (E) 1-meso300. With an increase in the modulator, the crystal size observed in the TEM image gradually increased. (F) Scanning electron microscopy image of 1-micro. (G) X-ray diffraction patterns of complete dried crystals of all above-mentioned sizes, compared with simulated patterns. The closed phase was observed under completely dried conditions for large crystals of 1-micro and 1-meso300. With decreasing crystal size, the contribution of the unusual open dried phase increased. The smallest crystal of 1-meso50 was mainly composed of the open dried phase.

In the intermediate crystal size (between 1-meso300 and 1-meso50), the contribution of the open dried phase increased with a decrease in the crystal size. We used thermogravimetric analysis to verify the lack of guest inclusion in the open dried phase of 1-meso50; weight loss did not occur before the decomposition temperature (280°C) (figs. S8 to S19). The meso-sized crystals, 1-meso50, allowed us to isolate the temporary phase as the open dried phase. Thermal treatment recovered the original closed phase from the open dried phase, which was characterized by variable temperature powder XRD measurements (figs. S20 and S21): (i) The open dried phase was converted into the closed phase by heating to 200°C; (ii) an intermediate structural phase was not observed; and (iii) the closed phase was maintained, even after cooling to room temperature. Thus, the open dried phase was metastable relative to the closed phase. This thermodynamic analysis was also supported by differential scanning calorimetry measurements for the open dried phase, in which we observed a broad exothermic peak for only the first scan but not for the second scan (fig. S22). We performed a solvent-mediated structural transformation to successfully reproduce the open dried phase: The closed phase of 1-meso50 was immersed in methanol, and the guest methanol molecules were then removed under vacuum (fig. S23). Hence, we demonstrated the MSME of 1-meso50.

The systematic production of size-controlled mesoscopic crystals of 1 allowed us to investigate the correlation between the crystal size and the sorption properties. We obtained the methanol adsorption isotherm of the conventional micrometer crystals of 1-micro at 30°C and observed the characteristic sharp uptake of molecules at P/P0 = 0.10, or gate-opening pressure (P/P0, relative pressure) (Fig. 4A and figs. S24 to S29). Using an environmentally controlled synchrotron XRD system (fig. S30), we determined that the sharp uptake corresponded to a structural transformation from the nonporous closed phase to the porous open phase in response to methanol accommodation (fig. S31). We then performed methanol adsorption measurements on the closed phase of the size-controlled mesoscopic crystals. As the crystal size decreased from 1-micro to 1-meso50, the gate-opening pressure shifted to a higher value. Environmentally controlled synchrotron XRD measurements on closed 1-meso50 revealed that the structural transformation occurred in a higher-pressure region and presented a wider pressure range (P/P0 = 0.14 to 0.30 for 1-meso50) than that of 1-micro (P/P0 = 0.10 to 0.17) (fig. S32). This experiment also suggested that the gate-opening pressure can be controlled by the crystal size.

Fig. 4

(A) Methanol adsorption isotherm at 303 K of the closed phase of 1-meso50 (blue), 1-meso60 (turquoise), 1-meso110 (green), 1-meso160 (yellow), 1-meso300 (orange), and 1-micro (red). Arrows represent the pressure regions in which the structural change from the closed phase to the open phase was observed for 1-micro (red) and 1-meso50 (blue). (B to D) Switchable methanol adsorption behavior of the shape-memory effect in 1-meso50 (298 K). (B) Isotherm of the open dried 1-meso50. (C) Isotherm of the closed 1-meso50 generated by heating of the open dried 1-meso50. (D) Second cycle of the isotherm of the closed 1-meso50 (i.e., isotherm of the reopened 1-meso50). STP, standard temperature and pressure.

We performed similar experiments on [Cu2(bdc)2(bpe)]n [2, bpe = 1,2-bis(4-pyridyl)ethylene], which is a twofold interpenetrated framework with larger unit cell parameters than those of 1 (fig. S43 to S56). The open dried phase was isolated in the metastable state when the crystals were downsized to ~700 by 700 by 80 nm3 (2-meso700) and interconvertible with the closed phase (figs. S58), which indicated the generation of a MSME. As the crystal size of 2 was decreased further (300 nm or less), the open dried phase could no longer be converted to the closed phase, even after heating to 200°C (fig. S57). Thus, the smaller crystals of 2-meso300 and 2-meso50 were no longer flexible but had become rigid, indicating a suppression of the structural mobility induced by crystal downsizing. These results suggest that framework flexibility could be controlled by crystal size. The emergence of an unprecedented framework rigidity in the smaller meso-sized crystals of 2 implies that the MSME was induced in the framework as an intermediate phenomenon between the elastic and the rigid frameworks achieved by crystal downsizing.

Regardless of the crystal size, the process of guest removal, which transforms the structure from the guest-included open phase to the closed phase, probably results in the formation of the open dried phase. In the micrometer-sized crystal, this phase was transient and easily converted to the more stable closed phase. Crystal downsizing apparently stabilized the transient state into a metastable state at room temperature. This stabilization is attributed to the thermodynamic and/or kinetic suppression of the phase transition from the open dried phase to the closed phase (fig. S59). The change in the relative thermodynamics between the closed and open dried phases is likely induced by an increase in the surface enthalpy contribution, which has been observed previously for condensed metal oxide systems (20).

However, we observed stabilization in the relatively large crystal size of 2-meso700, so kinetic suppression should also be considered. Kinetic suppression of the phase transition originates from the barrier between the two phases. We investigated the effect of crystal size on the kinetic suppression using the hysteretic sorption isotherm attributed to the barrier between the closed phase and the open phase (21). The methanol isotherm for 1-meso300 showed a larger hysteretic width compared with that of 1-micro; however, the center of the hysteretic loop was maintained (fig. S60), indicating that the energy barrier of the structural transformation was higher for the smaller crystals. The structural transformation described herein is cooperative and lattice-distortive, which are characteristics of martensitic transformations. The size-dependent kinetic suppression of martensitic materials (22) is often explained by a decrease in the number of lattice defects, which exist even in high-quality crystals. According to the heterogeneous nucleation and growth mechanism of the martensitic phase transformation, the defects provide sites with different nucleation potencies and cooperativity dominates growth (23). Based on the assumption that larger crystals contain more nucleation sites than smaller crystals, a statistical model attributed to the nucleation potency distribution explains the enhanced kinetic suppression of the phase transition by crystal downsizing (fig. S61) (24).

We used the closed and open dried phases as the initial states for the demonstration of switchable adsorption (25). The methanol adsorption isotherm of the open dried phase of 1-meso50 did not show gate-type sorption behavior but instead presented type I uptake, followed by desorption along the adsorption profile, which is characteristic sorption behavior generally observed in rigid microporous materials (Fig. 4B and fig. S34) (26). As supported by environmentally controlled XRD measurements, structural change did not occur during methanol adsorption (fig. S33). The pore shape was switched from the temporary open dried phase to the original closed phase by heating to 200°C. The resulting material showed adsorption with gate-type sorption behavior (Fig. 4C and fig. S35). The desorption isotherm of the closed phase was similar to that of the open dried phase, indicating that the open dried phase was regenerated after complete desorption. Indeed, the following adsorption isotherm again exhibited a type I profile and could be superimposed over that of the initial open dried phase (Fig. 4D and fig. S36). We further confirmed the repeatability of switching: The adsorption capacity of the open dried phase did not change at all, even after 20 cycles of the open/closed reversible transformation (figs. S37 to S39). We also observed similar adsorption properties (that is, the sorption change between the two distinct initial states) for CO2 (figs. S40 to S42). The induction of a MSME for both liquid- and gas-phase molecules could be exploited as intelligent functional materials that respond to the microscopic environmental changes.

Supplementary Materials

www.sciencemag.org/cgi/content/full/339/6116/193/DC1

Materials and Methods

Supplementary Text

Figs. S1 to S61

Table S1

References (2729)

References and Notes

  1. Supplementary materials are available on Science Online.
  2. Acknowledgments: Y.S. is grateful to the Japan Society for the Promotion of Science Research Fellowships for Young Scientists. The Institute for Integrated Cell-Material Sciences (iCeMS) is supported by World Premier International Research Initiative (WPI); Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan. The synchrotron x-ray experiments were performed at BL13XU in the SPring-8 under the Priority Nanotechnology Support Program administered by the Japan Synchrotron Radiation Research Institute (JASRI) (proposal no. 2011B1671). We also thank T. Asai and M. Maeda (TA Instruments, Japan) for the differential scanning calorimetry measurements. Crystallographic data for 1⊃ethanol, 1⊃methanol, 1⊃DMF, 1, and 2⊃ethanol have been deposited with the Cambridge Crystallographic Data Centre under reference nos. CCDC 863312 to 863316, respectively. These data can be obtained free of charge via www.ccdc.cam.ac.uk.
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